The present invention relates to a hologram recording sheet suitable for multicolor displays which is required to selectively diffract light of two or more wavelengths, to a holographic optical element such as a multicolor head-up display combiner, a multicolor display hologram, a heat-wave reflecting film that is effectively used on the windows of a car or building , a broad band of holographic filter, and a diffraction grating which, like the heat-wave reflecting film, diffracts light in a wide wavelength range, and to a process for producing such a holographic optical element.
So far, hologram sensitive materials have been widely used for holograms for ornamental, forgery-preventing, optical element or other purposes. Usually, a typical hologram sensitive material is made up of a PET substrate on which a hologram sensitive material 2 sensitive to green and red light is uniformly coated in the form of a single layer, as can be seen from FIG. 1. This arrangement is applied to a hologram optical element required to selectively reflect or diffract light of two or more wavelengths, esp., to a head-up display combiner designed to change color from site to site. In a typical combiner, a hologram sensitive material capable of diffracting light of plural wavelengths is produced on the entire surface thereof , thereby enabling all colors to be displayed.
In the conventional arrangement in which a hologram capable of displaying all colors is provided on the entire surface of the combiner, there is need of diffracting a plurality of light rays having different wavelengths. As shown in FIG. 2 by way of example, plural sets of interference fringes such as interference fringes 4 that diffract green light and interference fringes 5 that diffract red light must be produced in the thickness direction of the hologram. However, not only does this give rise to technical difficulties but also impose some considerable limitation on the hologram recording material to be used. In addition, this is responsible for inferior image quality such as ghost images.
One possible approach to solving this problem is the lamination of hologram sensitive materials sensitive to different wavelength regions. However, high coating technologies such as slide coating, slot coating and curtain coating are needed for the uniform lamination of many hologram sensitive materials with their interfaces kept constant, and much tie is taken to predetermine the required conditions etc. At the same time, this approach has a cost problem, because there is need of using an exclusive fountain head. Another approach is to use a so-called dry type of hologram recording material recently put forward by DU Pont or Government Industrial Research Institute, Osaka, which is designed to record interference fringes by the migration of the monomer contained therein. However, a grave problem with this is that the unrestricted migration of the monomer takes place through the interfaces; in other words, the pitch of the interference fringes cannot be obtained, as designed, although depending on what type of hologram-recording process is used. Notice that this is quite true when reliance is on sequential exposure.
On the other hand, attention is now paid to a multicolor display hologram having excellent effects on ornamentation and preventing forgery and a volume phase (Lippmann) type of hologram excellent in wavelength selectivity and having a profusion of three dimensional depth. A hologram combiner is one of such optical elements, and functions as a semi-transmitting image-formation element. FIG. 3(a) is a schematic that illustrates how to take a photograph of the image. Light from a laser 10 is split by a half-mirror 11 into two light beams, one traveling to one point of a lens 12, where it is converted to divergent rays and the other propagating to one point of a lens 13, where it is converted to divergent rays. These two light rays are incident on both sides of a volume phase type of hologram recording material 14, for instance, a photopolymer, so that they can interfere as a Lippmann hologram. This is a hologram combiner.
As shown in FIG. 3(b), such a hologram combiner 16 diffracts light leaving a display object 15 located in the vicinity of one divergent point for image-taking in the reflection direction, so that the diffracted image can emerge as if they came from a display object 15 located in the vicinity of the other divergent point for image-taking. The image-formation magnification and the image position are determined by the relative distances L and L′ between the divergent point for image-taking and the recording material. Hence, this hologram combiner functions to diffract only the light of the wavelength for recording or a wavelength having a specific relation thereto and transmit light of other wavelength, so that the image superposition or synthesis can be achieved. The optical elements represented by such a hologram combiner include a head-up display combiner.
Optical systems for recording and reconstructing various color display holograms will now be explained with reference to FIGS. 4 to 6.
FIG. 4 is a schematic of an optical system for recording and reconstructing a laser light reconstructing hologram. As shown in FIG. 4(a), a sensitive material 21 is trebly exposed to light at sequentially varying wavelengths R→G→B, so that light from an object 20 and reference light can interfere on a recording material 21 to record interference fringes. For reconstruction, the hologram 21 is illuminated by mixed (R+G+B) light from the same direction as the reference light, as shown in FIG. 4(b), so that the color hologram image of the object 20 can be observed at the original position.
FIG. 5 is a schematic representing an optical system for recording and reconstructing a rainbow hologram. At the first stage light from an object 20 and reference light interfere on three sensitive materials 21 at sequentially varying recording wavelengths R→G→B, so that three first holograms for R, G and B can be produced, as shown in FIG. 5(a). At the second stage, the first holograms are illuminated by light on the wave fronts conjugate to the reference waves, so that a real image of the object is reproduced at the original object position. Then, while a second sensitive material 23 is located at the position where the real image is reproduced and a slit is placed just in front of the first holograms, thereby keeping transverse parallax intact and eliminating longitudinal parallax, treble exposure is carried out at varying recording wavelengths R→G→B and with varying three first holograms for R, G and B, so that a second hologram is produced by interference with the reference light, as shown in FIG. 5(b). Upon reconstruction by white light on the wave fronts conjugate to the reference waves, an image having a profusion of three dimensional depth is produced in the lengthwise direction of the slit. If the observer's viewing position is moved vertically to this, the reconstructed image in different color can then be observed, as shown in FIG. 5c. 
FIG. 6 is a schematic representing an optical system for recording and reconstructing a Lippmann hologram. At the first stage, three first holograms for R, G and B are produced as in the case of the rainbow hologram (see FIG. 6(a)). At the second stage, a second sensitive material 23 is located at the position of the first hologram where a real image is reproduced, and is then illuminated by reference light from the opposite direction for treble exposure at varying recording wavelengths R→G→B, thereby producing a second hologram, as shown in FIG. 6b). Upon reconstruction by the illumination of the second hologram by the reference light and white light from the opposite direction, a color image is reproduced by the reflected and diffracted light, as shown in FIG. 6(c).
In addition, a surface relief type of hologram can be produced by copying a mold having surface asperities in the form of interference fringes.
To achieve displays in multicolor by the conventional method, plural sets of interference fringes such as interference fringes 30 diffracting blue light, interference fringes 31 diffracting green light and interference fringes 32 diffracting red light must be superposed on the same hologram in the thickness direction in the case of a Lippmann hologram, as shown in FIG. 7(a). In the case of laser light reconstructing and rainbow holograms, plural sets of interference fringes such as interference fringes 30 diffracting blue light, interference fringes 31 diffracting green light and interference fringes 32 diffracting red light must be similarly formed, as shown in FIG. 7(b). Not only does this give rise to technical difficulties, but also places some considerable limitation on the hologram recording material to be used, because the sensitive material is required to be sensitive to plural colors. There is also a diffraction efficiency drop. Moreover, this is responsible for inferior image quality such as large noises, for instance, ghost images.
So far, heat-wave reflecting films have been put on the windows of cars or buildings so as to control a rise in the inside temperature. A typical conventional heat-wave reflecting film has its surface designed to reflect heat waves in a preset, wavelength range. To reflect heat waves of wavelength longer than the preset wavelength, there is need of adding to each site of the film surface a function capable of reflecting heat waves in a desired wavelength region. This has heretofore been achieved by using a dielectric material and a metal or metal oxide thin film, etc.
However, conventional heat-wave reflecting films such as deposited films, except holograms, when used to reflect heat waves in a wide wavelength range, result unavoidably in a lowering of the transmission of visible light, because they are likely to reflect or absorb visible light. Diffraction gratings, although they may somehow be used as heat-wave reflecting films, will make it difficult to reflect heat waves in a wide wavelength range due to their wavelength selectivity. This problem may possibly be solved by superposing layers diffracting different wavelength regions in the film thickness direction. However, a problem with this superposition is that the light diffracted by one layer is further diffracted by another layer, thus making it very difficult to achieve any effective reflection of heat waves.
When interference fringes of a given pitch is recorded on a photosensitive material, the diffraction efficiency shows a peak with respect to the wavelength determined by that pitch. Hence, a photosensitive material with interference fringes of a certain pitch recorded thereon can be used as an optical filter, because its reflectivity with respect to light of a given wavelength can be increased. So far, this has been extended to a heat-wave reflecting film having increased reflection properties with respect to the infrared region, for instance. Also, methods of disturbing interference fringes by material treatments, thereby making the band width of the reflection wavelength region wider, have been put forward.
However, it is extremely difficult to produce a wide band width of titer such as a solar reflector with the use of a diffracting grating with interference fringes recorded thereon. In particular, a grave problem with widening band width by conventional material treatments is that it is extremely difficult to regulate the reflection wavelength region to a specific region.
one of well-known diffraction gratings is a volume hologram produced by recording interference fringes on a film made up of photopolymer, dichromated gelatin, silver salt or the like by interference of light. However, this volume hologram has a narrow diffraction wavelength range; no volume hologram having a wide wavelength range is available as yet. Such a diffraction grating, when used in the form of a heat-wave reflecting film or the like, is required to have a diffraction wavelength range of at least a few 100 nm.